azure: detect vnet migration via netlink media change event

Replace Azure pre-provision polling on IMDS with a blocking call
which watches for netlink link state change messages.  The media
change event happens when a pre-provisioned VM has been activated
and is connected to the users virtual network and cloud-init can
then resume operation to complete image instantiation.

1 files changed, 250 insertions, 0 deletions

diff --git a/cloudinit/sources/helpers/netlink.py b/cloudinit/sources/helpers/netlink.py
new file mode 100644
index 00000000..d377ae3d
--- /dev/null
+++ b/cloudinit/sources/helpers/netlink.py
@@ -0,0 +1,250 @@
+# Author: Tamilmani Manoharan <tamanoha@microsoft.com>
+#
+# This file is part of cloud-init. See LICENSE file for license information.
+
+from cloudinit import log as logging
+from cloudinit import util
+from collections import namedtuple
+
+import os
+import select
+import socket
+import struct
+
+LOG = logging.getLogger(__name__)
+
+# http://man7.org/linux/man-pages/man7/netlink.7.html
+RTMGRP_LINK = 1
+NLMSG_NOOP = 1
+NLMSG_ERROR = 2
+NLMSG_DONE = 3
+RTM_NEWLINK = 16
+RTM_DELLINK = 17
+RTM_GETLINK = 18
+RTM_SETLINK = 19
+MAX_SIZE = 65535
+RTA_DATA_OFFSET = 32
+MSG_TYPE_OFFSET = 16
+SELECT_TIMEOUT = 60
+
+NLMSGHDR_FMT = "IHHII"
+IFINFOMSG_FMT = "BHiII"
+NLMSGHDR_SIZE = struct.calcsize(NLMSGHDR_FMT)
+IFINFOMSG_SIZE = struct.calcsize(IFINFOMSG_FMT)
+RTATTR_START_OFFSET = NLMSGHDR_SIZE + IFINFOMSG_SIZE
+RTA_DATA_START_OFFSET = 4
+PAD_ALIGNMENT = 4
+
+IFLA_IFNAME = 3
+IFLA_OPERSTATE = 16
+
+# https://www.kernel.org/doc/Documentation/networking/operstates.txt
+OPER_UNKNOWN = 0
+OPER_NOTPRESENT = 1
+OPER_DOWN = 2
+OPER_LOWERLAYERDOWN = 3
+OPER_TESTING = 4
+OPER_DORMANT = 5
+OPER_UP = 6
+
+RTAAttr = namedtuple('RTAAttr', ['length', 'rta_type', 'data'])
+InterfaceOperstate = namedtuple('InterfaceOperstate', ['ifname', 'operstate'])
+NetlinkHeader = namedtuple('NetlinkHeader', ['length', 'type', 'flags', 'seq',
+                                             'pid'])
+
+
+class NetlinkCreateSocketError(RuntimeError):
+    '''Raised if netlink socket fails during create or bind.'''
+    pass
+
+
+def create_bound_netlink_socket():
+    '''Creates netlink socket and bind on netlink group to catch interface
+    down/up events. The socket will bound only on RTMGRP_LINK (which only
+    includes RTM_NEWLINK/RTM_DELLINK/RTM_GETLINK events). The socket is set to
+    non-blocking mode since we're only receiving messages.
+
+    :returns: netlink socket in non-blocking mode
+    :raises: NetlinkCreateSocketError
+    '''
+    try:
+        netlink_socket = socket.socket(socket.AF_NETLINK,
+                                       socket.SOCK_RAW,
+                                       socket.NETLINK_ROUTE)
+        netlink_socket.bind((os.getpid(), RTMGRP_LINK))
+        netlink_socket.setblocking(0)
+    except socket.error as e:
+        msg = "Exception during netlink socket create: %s" % e
+        raise NetlinkCreateSocketError(msg)
+    LOG.debug("Created netlink socket")
+    return netlink_socket
+
+
+def get_netlink_msg_header(data):
+    '''Gets netlink message type and length
+
+    :param: data read from netlink socket
+    :returns: netlink message type
+    :raises: AssertionError if data is None or data is not >= NLMSGHDR_SIZE
+    struct nlmsghdr {
+               __u32 nlmsg_len;    /* Length of message including header */
+               __u16 nlmsg_type;   /* Type of message content */
+               __u16 nlmsg_flags;  /* Additional flags */
+               __u32 nlmsg_seq;    /* Sequence number */
+               __u32 nlmsg_pid;    /* Sender port ID */
+    };
+    '''
+    assert (data is not None), ("data is none")
+    assert (len(data) >= NLMSGHDR_SIZE), (
+        "data is smaller than netlink message header")
+    msg_len, msg_type, flags, seq, pid = struct.unpack(NLMSGHDR_FMT,
+                                                       data[:MSG_TYPE_OFFSET])
+    LOG.debug("Got netlink msg of type %d", msg_type)
+    return NetlinkHeader(msg_len, msg_type, flags, seq, pid)
+
+
+def read_netlink_socket(netlink_socket, timeout=None):
+    '''Select and read from the netlink socket if ready.
+
+    :param: netlink_socket: specify which socket object to read from
+    :param: timeout: specify a timeout value (integer) to wait while reading,
+            if none, it will block indefinitely until socket ready for read
+    :returns: string of data read (max length = <MAX_SIZE>) from socket,
+              if no data read, returns None
+    :raises: AssertionError if netlink_socket is None
+    '''
+    assert (netlink_socket is not None), ("netlink socket is none")
+    read_set, _, _ = select.select([netlink_socket], [], [], timeout)
+    # Incase of timeout,read_set doesn't contain netlink socket.
+    # just return from this function
+    if netlink_socket not in read_set:
+        return None
+    LOG.debug("netlink socket ready for read")
+    data = netlink_socket.recv(MAX_SIZE)
+    if data is None:
+        LOG.error("Reading from Netlink socket returned no data")
+    return data
+
+
+def unpack_rta_attr(data, offset):
+    '''Unpack a single rta attribute.
+
+    :param: data: string of data read from netlink socket
+    :param: offset: starting offset of RTA Attribute
+    :return: RTAAttr object with length, type and data. On error, return None.
+    :raises: AssertionError if data is None or offset is not integer.
+    '''
+    assert (data is not None), ("data is none")
+    assert (type(offset) == int), ("offset is not integer")
+    assert (offset >= RTATTR_START_OFFSET), (
+        "rta offset is less than expected length")
+    length = rta_type = 0
+    attr_data = None
+    try:
+        length = struct.unpack_from("H", data, offset=offset)[0]
+        rta_type = struct.unpack_from("H", data, offset=offset+2)[0]
+    except struct.error:
+        return None  # Should mean our offset is >= remaining data
+
+    # Unpack just the attribute's data. Offset by 4 to skip length/type header
+    attr_data = data[offset+RTA_DATA_START_OFFSET:offset+length]
+    return RTAAttr(length, rta_type, attr_data)
+
+
+def read_rta_oper_state(data):
+    '''Reads Interface name and operational state from RTA Data.
+
+    :param: data: string of data read from netlink socket
+    :returns: InterfaceOperstate object containing if_name and oper_state.
+              None if data does not contain valid IFLA_OPERSTATE and
+              IFLA_IFNAME messages.
+    :raises: AssertionError if data is None or length of data is
+             smaller than RTATTR_START_OFFSET.
+    '''
+    assert (data is not None), ("data is none")
+    assert (len(data) > RTATTR_START_OFFSET), (
+        "length of data is smaller than RTATTR_START_OFFSET")
+    ifname = operstate = None
+    offset = RTATTR_START_OFFSET
+    while offset <= len(data):
+        attr = unpack_rta_attr(data, offset)
+        if not attr or attr.length == 0:
+            break
+        # Each attribute is 4-byte aligned. Determine pad length.
+        padlen = (PAD_ALIGNMENT -
+                  (attr.length % PAD_ALIGNMENT)) % PAD_ALIGNMENT
+        offset += attr.length + padlen
+
+        if attr.rta_type == IFLA_OPERSTATE:
+            operstate = ord(attr.data)
+        elif attr.rta_type == IFLA_IFNAME:
+            interface_name = util.decode_binary(attr.data, 'utf-8')
+            ifname = interface_name.strip('\0')
+    if not ifname or operstate is None:
+        return None
+    LOG.debug("rta attrs: ifname %s operstate %d", ifname, operstate)
+    return InterfaceOperstate(ifname, operstate)
+
+
+def wait_for_media_disconnect_connect(netlink_socket, ifname):
+    '''Block until media disconnect and connect has happened on an interface.
+    Listens on netlink socket to receive netlink events and when the carrier
+    changes from 0 to 1, it considers event has happened and
+    return from this function
+
+    :param: netlink_socket: netlink_socket to receive events
+    :param: ifname: Interface name to lookout for netlink events
+    :raises: AssertionError if netlink_socket is None or ifname is None.
+    '''
+    assert (netlink_socket is not None), ("netlink socket is none")
+    assert (ifname is not None), ("interface name is none")
+    assert (len(ifname) > 0), ("interface name cannot be empty")
+    carrier = OPER_UP
+    prevCarrier = OPER_UP
+    data = bytes()
+    LOG.debug("Wait for media disconnect and reconnect to happen")
+    while True:
+        recv_data = read_netlink_socket(netlink_socket, SELECT_TIMEOUT)
+        if recv_data is None:
+            continue
+        LOG.debug('read %d bytes from socket', len(recv_data))
+        data += recv_data
+        LOG.debug('Length of data after concat %d', len(data))
+        offset = 0
+        datalen = len(data)
+        while offset < datalen:
+            nl_msg = data[offset:]
+            if len(nl_msg) < NLMSGHDR_SIZE:
+                LOG.debug("Data is smaller than netlink header")
+                break
+            nlheader = get_netlink_msg_header(nl_msg)
+            if len(nl_msg) < nlheader.length:
+                LOG.debug("Partial data. Smaller than netlink message")
+                break
+            padlen = (nlheader.length+PAD_ALIGNMENT-1) & ~(PAD_ALIGNMENT-1)
+            offset = offset + padlen
+            LOG.debug('offset to next netlink message: %d', offset)
+            # Ignore any messages not new link or del link
+            if nlheader.type not in [RTM_NEWLINK, RTM_DELLINK]:
+                continue
+            interface_state = read_rta_oper_state(nl_msg)
+            if interface_state is None:
+                LOG.debug('Failed to read rta attributes: %s', interface_state)
+                continue
+            if interface_state.ifname != ifname:
+                LOG.debug(
+                    "Ignored netlink event on interface %s. Waiting for %s.",
+                    interface_state.ifname, ifname)
+                continue
+            if interface_state.operstate not in [OPER_UP, OPER_DOWN]:
+                continue
+            prevCarrier = carrier
+            carrier = interface_state.operstate
+            # check for carrier down, up sequence
+            isVnetSwitch = (prevCarrier == OPER_DOWN) and (carrier == OPER_UP)
+            if isVnetSwitch:
+                LOG.debug("Media switch happened on %s.", ifname)
+                return
+        data = data[offset:]
+
+# vi: ts=4 expandtab